How to determine shear sense from flanking folds
Flanking folds can form by the two main groups of mechanisms mentioned above, and by polyphase deformation. Andersonian flanking folds are most useful as shear sense indicators, since they are better understood and more predictable in their development. Basically, the geometry of the folds that form depend on initial orientation of the faults with respect to the HE (the angle α), and the flow conditions. Even these two basic parameters can give rise to a bewildering array of flanking fold types (Passchier, 2001; Grasemann and Stüwe, 2001; Grasemann et al., 2003; Exner et al., 2004; Grasemann et al., 2005; Wiesmayr and Grasemann, 2005; Coelho et al., 2005; Kocher and Mancktelow, 2006; Gomez et al. 2007). Grasemann et al. (2005) even lamented that flanking folds are useless as shear sense indicators since they occur in mirror pairs for opposite shear sense. Gomez et al. (2007) suggested to use groups of flanking folds in different stages of development, but the obvious problem is to decide whether flanking folds of different shape really belong together.
The situation may not be as desperate as suggested by the literature, though. First of all, most shear zones with undeformed wall rocks must form in a flow regime close to simple shear. Secondly, the range of possible initial orientation of fractures may be limited. From the orientation of the stress field, fractures will most likely develop in tension, parallel to σ1 of stress or at a small angle to σ1: in simple shear, this implies fractures between 20°-80° inclined to the flow plane (Fig. 8, inset). Gently dipping fractures will lie parallel to HE, or develop into normal shear bands in this case, but the steep fractures will develop a reversed shear sense and rotate in the shear direction; these are the ones that can develop shear band-shaped flanking folds for shallow angles, and hook shapes for steep angles. The possible range of development of such structures is illustrated in Fig. 8.
Figure 8. Possible range of flanking folds and their evolution within sinistral shear.
a) Shear bands with normal slip. b) Shear band with reverse slip that evolves into hook shaped flanking fold with reverse slip along shallow CE. c) Hooked shape flanking fold with reverse slip along steep CE, could evolve into normal slip with progressive deformation. d) Flanking folds along a rigid CE.
Shear bands with normal slip on shallow fractures may not rotate enough to develop into hook-shaped flanking folds (Movie 5; Fig. 8a). However, those on faults with reverse slip will develop into a shear band geometry and gradually rotate to form tight hook shaped flanking folds (Movie 4; Fig 8b, c). Only originally steep faults will develop hook-shaped flanking with reverse slip from a start (Fig. 8c). Since these are much more common than shear band shapes with reverse slip, most active fractures in shear zones probably start a at steep angles to the foliation.
If flanking folds form early during the development of a ductile shear zone, they may gradually develop into closed hook shaped folds, while the shear sense on the fault may reverse during deformation (movie 3; Fig. 8b, c). This was first shown by Exner et al. (2002) and is confirmed by our experimental results presented above (movie 3). As a result, the reverse slip of hook-type flanking folds can transform to normal slip, after which the flanking folds cannot be distinguished from those that would form by a fold train that is cut by a late fault. Fortunately, the vergence of the folds is similar for all these different scenarios. Object related flanking folds formed from original steep orientations of the CE will also develop the same vergence as Andersonian ones. As a result, flanking folds can usually be used as shear sense indicators.